1. Field of the Invention
The present invention relates to an optical device such as an exchangeable lens unit comprising, for example, focus lens groups and a diaphragm mechanism.
2. Description of the Related Art
An optical system is available, which comprises focus lens groups and which can achieve macro photography. As is known, the F value changes as the focus lens groups move in the optical system. The change in the F value results from an optically inherent phenomenon that the luminous energy at image plane decreases as the photographing magnification is raised.
In this regard, Jpn. Pat. Appln. KOKAI Publication No. 11-231210 discloses a technique of keeping the F value unchanged even if the focus lens groups move. The optical system of Jpn. Pat. Appln. KOKAI Publication No. 11-231210 has a lens barrel in which the diaphragm is opened and closed by a cam mechanism driven as the focus lens groups move.